1. Field of the Invention
The present invention relates to color proofing films. More particularly, the invention pertains to a peel-apart, single sheet color proofing system capable of producing negative images upon exposure to infrared laser radiation and peel development.
2. Description of the Related Art
In the graphic arts, it is desirable to produce a three or more color proof to assist in correcting a set of color separation films prior to using them to produce printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired halftone image. Visual examination of a color proof should show the color rendition to be expected from press printing using the color separations and any defects on the separations which might need to be altered before making the printing plates.
Color proofing sheets for multicolored printing can be made by using a printing press or proof press. This requires that all of the actual printing steps be performed. Therefore, this conventional method of color proofing is costly and time consuming. Color proofing films can also be used to simulate printing. There are two general types of color proofing methods, namely the overlay type and the single sheet type.
In the overlay type of color proofing method, an independent transparent plastic support is used for producing an image of each color separation film by applying a photosensitive solution of the corresponding color. A plurality of such supports carrying images of the corresponding colors are then superimposed upon each other over a white sheet to produce a color proofing composite. The primary advantage of the overlay method is that proofs can be made quickly and can serve as a progressive proof by combining any two, three or more colors in register. However, this type of color proofing method has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet. As a result, the impression of the color proofing composite thus prepared is vastly different from that of copies actually obtained with conventional printing presses and with proof presses. Examples of such overlay approaches are contained in U.S. Pat. Nos. 3,136,637; 3,211,553; and 3,326,682.
In the single sheet type of color proofing method, a color proofing sheet is prepared by successively producing images of different colors from different color separation films on a single receiver sheet. This can be accomplished by sequentially applying colorants or colored, photosensitive layers to a single opaque support. This method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system. Examples of such single sheet approaches are contained in U.S. Pat. Nos. 3,574,049; 3,671,236; 4,260,673; 4,366,223; 4,650,738; 4,656,114; and 4,659,642.
Peel-apart image forming elements are used in imaging processes because they are very convenient. After exposure, the element is merely exposed and peeled apart, separating exposed areas from unexposed areas. The peel development process is completely dry, with no wet processing required.
There are various methods of creating a peel developed image on a receiver sheet, including ultraviolet imaging and thermal imaging methods. Thermal imaging involves exposure to infrared radiation. For example, U.S. Pat. No. 5,326,619 describes an infrared laser induced thermal imaging method involving a transfer of thermal material from a donor element to a receiver. The thermal element is a color layer and includes a black metal layer and a gas-producing polymer layer. U.S. Pat. No. 5,697,300 teaches a thermal imaging system wherein the transfer material reduces in viscosity to a flowable state. After peel development, the exposed material transfers in the flowable state to a receiver sheet and is then fused to the receiver sheet in a subsequent fusing step.
Other thermal image forming processes have been employed where the bonding strength between a thermal layer and a color layer is increased through exposure. U.S. Pat. No. 5,352,562 teaches an image forming layer is exposed to light and the bonding strength between the image forming layer and a light-heat conversion layer is increased. After peel development the unexposed image remains on a receiver sheet. WO 93/03928 teaches a peel-apart, laser-induced thermal imaging process wherein exposure to infrared radiation increases the bond strength between an infrared radiation absorbing element and a support, leaving unexposed areas on a receiver sheet after peel development.
It has now been unexpectedly found that a thermal imageable element sequentially having a substrate, a crosslinked thermal transfer layer, a color layer and a thermoplastic adhesive layer will show a reduced bond strength between the crosslinked thermal transfer layer and a color layer at exposed areas after exposure to infrared radiation resulting in the transfer of underlying portions of the colored layer to the adhesive layer on the receiver sheet without any transfer of the crosslinked thermal transfer layer. The unexposed areas remain on the crosslinked thermal transfer layer on the substrate.
The crosslinked thermal transfer layer comprises carbon particles in a crosslinked polymer which is capable of converting infrared laser radiation to thermal energy. This layer is heat crosslinked after coating to withstand solvent attack of subsequent coating operations of the color layer. Since carbon particles are trapped in the crosslinked thermal transfer layer, it can not be rubbed off the surface of the film during the coating operation, and no ablation transfer of carbon particles is possible when the film is infrared imaged. The density of this crosslinked thermal transfer layer is controlled so that the conversion of infrared laser radiation to thermal energy is most efficient. The adhesion between the crosslinked thermal transfer layer and the color coats can be controlled by varying the crosslinking agents, incorporating additional polymeric binder, or additives.